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\V. B. No. M9 



U. S. DEPARTMENT OF AGRICULTURE, 



WEATHER BUREAU. 

BULLETIN V. 



FROST DATA OF THE UNITED STATES; 

AND LENGTH OF THE CROP-GROWING SEASON, 

AS DETERMINED FROM THE AVERAGE 

OF THE LATEST AND EARLIEST 

DATES OF KILLING FROST. 



By P. C. DAY, Chief of Climatological Division. 



Prepared under the direction of WILLIS L. MOOTtE, Chief of Weather Bureau. 




I 
WASHINGTON : 

AVE A THEE BUEEAU. 

1911. 



iyiOAiW' 



W. B. No. 149. 



U..S. DEPARTMENT OF AGRICULTURE, 

WEATHER BUREAU. 

BULLETIN V. 



FROST DATA OF THE UNITED STATES; 

AND LENGTH OF THE CROP-GROWING SEASON, 

AS DETERMINED FROM THE AVERAGE 

OF THE LATEST AND EARLIEST 

DATES OF KILLING FROST. 



By P. C. DAY, Chief of Climatological Division. 



Prepared under the direction of WILLIS L. MOORE, Chief of Weather Bureau. 




WASHINGTON : 

WEATHER BUREAU. 

1911. 






# T)3 



FROST DATA AND LENGTH OF THE CROP-GROWING SEASON. 



SOURCE OF DATA. 



The tabulation of statistics regarding the occurrence of frosts in the United Stales lias in the 
past been confined in the main to the data collected at the regular telegraphic reporting stations of 
the Weather Bureau, something like 150 in number, with substantially continuous records, located 
principally in the larger cities and towns. 

On account of the many agencies operating in the cities that directly affect the local conditions, 
especially as to temperature and the occurrence of frost, it is clear that the records from these points 
do not represent the real conditions that prevail in the open country where the staple crops are 
grown. 

To secure data that would show more nearly the actual conditions that prevail in the fields, 
orchards and gardens, the most extensive compilation of frost data yet undertaken by the Weather 
Bureau has been accomplished and the results have been spread upon the accompanying charts. 

The data from approximately one thousand of our cooperative stations having the longest records, 
usually from about 10 to 30 years, except in the more newly settled localities of the West w hen- 
records for shorter periods only are available, have been summarized', and the local conditions clue 
to physical environments brought out in much greater detail than has heretofore been attempted. 

These charts, being based upon the results of observations made in the open country and there- 
fore not subject to the artificial conditions prevailing in the large cities where the regular stations 
of the Bureau are mainly located, differ from any that have appeared in the past in that areas hav- 
ing peculiar climatic features not heretofore shown on such charts are now clearly set forth. 

IMPORTANCE OF FROST STATISTICS. 

The increasing cost of farm lands, due to the rapid settlement of the country, and the growing 
demand for foodstuffs from the great centers of population make it necessary that greater returns be 
obtained from the soil than formerly. This in turn calls for the exercise of more care in the selec- 
tion of the crops best suited to the several localities and for additional knowledge as to the chief 
climatic factors in order that the maximum possible return shall be obtained both from the smallest 
amount of land and with the minimum of effort. 

The growing importance of fruit raising in all portions of the country and the tendency toward 
intensive cultural methods for all crops and the effort made to extend the field of profitable growt h 
of new and important varieties of farm products, necessitate the most careful consideration of the 
several factors that make for the success or failure of such efforts. 

Probably no factor in the study of climate from the standpoint of the agriculturist should be 
given more consideration than the average length of the growing season. This is the key to an 
actual knowledge as to the possibilities of success or failure in the production of crops, since in prac- 
tically all portions of the United States agricultural products are menaced by frost at some period 
of their growth. 

Modern devices for the protection of crops from frost at critical periods have clone much and 
will continue in larger measure to extend the field of profitable culture for many products into 
regions beyond the natural habitat of the plant; but much loss has been occasioned by efforts to 
extend such operations into fields which a careful study of the climate would have revealed as 



unsuitable on the average for such extension. It may also be assumed that such a study would 
make it possible to still farther extend the area of profitable cultivation of many classes of pro- 
duels. 

CHARTS PREPARED. 

The charts presented herewith show graphically: 

I. The average dale of the last killing frost in spring. 
II. The average date of the first killing frost in autumn. 
III. The latest date on which a killing frost has occurred in spring. 
IV. The earliest date on which a killing frost has occurred in autumn. 

V. The average length of the crop growing season, days, being the number of clays between 

the average date of the last killing frost in spring and the average date of the first 

killing frost in autumn. 

Over the districts east of the Rocky Mountains lines have been drawn through approximately 

identical dates for each 5 or 6 day period tor the charts of average dales of killing frost, spring and 

autumn, but in the two charts of extreme dales the periods cover 10 or 11 days, as the dates were too 

diverse to admit of a closer approximation. 

The chart showing the average length, in days, of the crop growing season was prepared from a 
somewhat different list of stations than was used for the charts of average dates of frost, hence an 
actual determination of the length of the season from Charts 1 and II might differ a few days from 
the data shown on Chart V. 

West of the Rocky Mountains il was not possible, on account of the diversified topography, to 
draw lines corresponding to those to the eastward, but the actual dales and other data are given for 
numerous points. Much diversity in these data is apparent and to be expected on account of the 
varying elevations of the different points of observation and their location with reference to the 
great mountain masses, the direction of the air drainage, etc. 

SPECIAL CONDITIONS AND THEIR INFLUENCE ON FROST FORMATION. 

In the elevated mountain districts and on some of the higher plateaus of the western country 
freezing temperatures are liable to occur in all months of the summer, but their effect on vegetation 
is not so pronounced as in the districts east of the mountains. 

Cool nights are a feature of all arid regions, due to the intense radiation made possible by the 
generally clear skies and the lack of moisture in the atmosphere. As a result of these conditions 
the temperature in the early morning hours may frequently reach the freezing point but its contin- 
uance may trot be for a length of time sufficient to injure the plant structure; in fact, owing to the 
dryness of the air, frost does not always form with a temperature of 32° or even several degrees 
lower, and in addition plant life subjected to such variations in temperature becomes more hardy and 
lower temperatures are required to cause serious injury. On the other hand, in the more humid 
regions the radiation at night is less rapid, the nights as a rule are not so markedly cold, plant life 
is less hardy, frost forms readily at the freezing point, the same degree of cold is often protracted 
over much longer periods of time, and vegetation is therefore more seriously affected. 

Throughout the entire mountain and plateau regions there are protected valleys and slopes 
where the influence of topography on air drainage is sufficient to considerably modify the effect of 
latitude in the distribution of temperature, and plants and fruits native to localities much farther 
south may be grown with little fear of injury from frost. There are also localities where on account 
of the drainage of air from high, snow-covered mountains frosts are more liable to occur than in 
other localities in the same latitude not so exposed. 

Consideration of these points is essential to the successful locating of sites for orchards and gar- 



5 

dens in all hilly and mountainous regions, il being possible to find belts on the sides of mountains 
or high Mills where plant growth begins several weeks earlier in the spring than al the tops of the 
hills or in the valleys below: and likewise in autumn frosts arc delayed to the same extent, thus 
lengthening the crop growing season in these belts several weeks. 

The influence of large bodies of water also in tempering the severity of frosts is clearly broughl 
out in these charts, particular attention being invited to the territory surrounding the Greal Lakes 
whore at nearby points the length of t he frost less season may differ by from 'Jo to 50 days. Also along 
the Atlantic Coast in Southern New England, over Long Island and in the vicinity of Chesapeake 
Hay the growing season is as long as in Tennessee, hundreds of miles to the southward. 

Likewise may be seen the increased probability of frost and the consequent shortening of t he 
growing season on the elevated level plateaus, notably in the Berkshire Hills of New England, over 
the plateau of western New York and north central Pennsylvania, in the Cumberland Plateau and 
in the highlands of Wisconsin and northern Michigan. 



Digitized by the Internet Archive 
in 2010 with funding from 
The Library of Congress 



http://www.archive.org/details/frostdataofuniteOOdayp 



Chart I.— THE AVERAGE DATE OF THE LAST KILLING FROST IN SP1 




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Chart II.— THE AVERAGE DA TE OF THE FIRST KILLING FROST IN A 




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Chart Y— THE AVERAGE LENGTH OF THE CROP-GROWING SEASON, 



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